Generic/JumbledIncrementer: fix non-problematic bug and improve test coverage #1421

Job	Run time
Build Phar on PHP: 8.0	39s
Coverage: 5.4	17m 16s
Coverage: 7.2 with custom ini settings	4m 1s
Coverage: 8.3	3m 17s
PHP: 5.4	1m 54s
PHP: 5.5	4m 14s
PHP: 5.6	3m 30s
PHP: 7.0	1m 42s
PHP: 7.1	1m 4s
PHP: 7.2	1m 1s
PHP: 7.3	48s
PHP: 7.4	50s
PHP: 8.0	55s
PHP: 8.1	49s
PHP: 8.2	1m 9s
PHP: 8.3	28s
PHP: 8.4	53s
PHP: 5.5 with custom ini settings	3m 45s
PHP: 7.0 with custom ini settings	43s
coveralls-finish	2s
	49m 0s

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